In an electric power steering device for vehicles, a speed reduction gear mechanism is installed between an electric motor and a steering shaft because the electric motor having relatively high revolutions and low torque is used. Speed reduction gear mechanisms with the use of a spur gear and other gears are known as the speed reduction gear mechanism. However, the use of a well-known worm speed reduction gear mechanism formed of a worm and a worm wheel is more typical because a larger speed reduction ratio can be obtained by a set of them, and so forth.
Such a worm speed reduction gear mechanism (hereinafter, simply referred to as reduction gear) is constructed from the worm that is a drive gear connected to a rotating shaft of an electric motor and a worm wheel meshed with the worm.
When both of the worm and the worm wheel in such a reduction gear are made of metal, there is a disadvantage that uncomfortable sounds such as rattling sound and vibration sound are generated when the steering wheel is handled. When the worm is made of metal, this problem has been conventionally solved by using a worm wheel provided with and formed integrally with a teeth portion made of a synthetic resin, in which a blank disc made of a synthetic resin material is integrally formed on the outer periphery of a metal hub, i.e. a metal core, as a worm wheel and teeth are formed on the periphery of the blank disc by cutting and other means, to suppress the generation of uncomfortable sounds such as rattling sound and vibration sound.
For the material of the resin part of the above-described resin gear, MC (monomer-cast) nylon (trademark) not containing a reinforcing material, polyamide 6, polyamide 66, and polyamide 46 that are blended with a reinforcing fiber material such as glass fiber, and the like are used in view of fatigue resistance, dimensional stability, and product cost. Generally, commercially available polyamide 6, polyamide 66, polyamide 46, and the like contain glass fiber having a diameter of ca. 10 μm or ca. 13 μm (refer to Japanese Examined Patent publication No. H06-60674 (60674/1994)).
In a worm speed reduction mechanism used in an electric power steering device, the worm is supported by two ball bearings, and a space between these two ball bearings is filled with grease for lubrication between the metal worm and the teeth of the worm wheel that is a resin gear. Generally, grease making use of mineral oil and poly-α-olefin oil in consideration of thermal resistance is used as a base oil.
Further, a damper made of rubber that allows not only a pre-load to be applied to the ball bearings arranged on both ends of the worm but also, when a small kick back is input from a tire side, information on the kickback to be transmitted only to a steering wheel by means of shifting the worm longitudinally so that a motor might not be influenced by a rotation force is sometimes attached. Usually for the rubber material, acryl rubber represented by ethylene acryl rubber having a small compression set is most generally used.
However, electric power steering devices have recently come to be used for from a light car to a car carrying a class of engine having a piston displacement of 1,000 cc to 1,500 cc, and the electric power steering devices have been high-powered. In accordance with this, PV value, the product of the contact surface pressure P at the resin gear portion and the peripheral velocity V, has become larger.
As a consequence of this, it has been found that the conventionally used grease that utilizes poly-α-olefin oil as abase oil is not effective enough for the lubrication condition between the driven gear (worm wheel) and the drive gear (worm). In addition, MC (monomer-cast) nylon (trademark) and generally commercially available polyamide resins containing glass fiber having a diameter of ca. 10 μm or ca. 13 μm have not sufficed for wear resistance at a high PV value.
As the result, there has been a problem that temporary shortage of oil film occurs due to long operation of an electric power steering device at a high PV value and wearing of tooth flank of the gear gradually progresses, thereby increasing the risk of backlash at the meshing portions between the driven gear (worm wheel) and the drive gear (worm). Thus, it is expected that steering sense worsens and unusual noises (rattling sounds) occur. Further, there has also been a fear that the whole electric power steering device becomes nonfunctional due to deformation of the gear, and in some cases, breakage of the gear.
In addition, there has been a possibility that the polyamide resins described above are highly water-absorptive in spite of being excellent in fatigue resistance and absorb water to swell the teeth portion of the gear of the worm wheel, spaces present between the worm and the worm wheel at an early stage after the production disappear, and the teeth portion presses the worm by further swelling.
When the worm wheel swells as just described, steering becomes heavier because frictional resistance between the worm and the worm wheel becomes larger, and the gear portion is worn out or broken due to an increase of pressure on the gear portion and an increase of frictional resistance, thereby giving rise to a failure of the electric power steering device.
The object of the present invention is to solve the above problems and to provide an electric power steering device, in which grease containing a wax to prevent a gear portion from being worn out and broken as well as to exert an efficient lubricating effect during a high temperature operation is used between a driven gear (worm wheel) and a drive gear (worm) and a resin gear suitable for power transmission of which dimensional change due to water absorption is suppressed by making up a resin part with a polyamide resin containing glass fiber finer than one conventionally used as a reinforcing fiber material and thus being free from fear of wear or breakage of the gear portion is used in the speed reduction gear mechanism constituting a power assist portion, and a resin gear used for the electric power steering device.